Power semiconductor switch

ABSTRACT

A SiC JFET that includes a plurality of trenches formed in a SiC semiconductor body of one conductivity each trench having a region of another conductivity formed in the bottom and sidewalls thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. patent application Ser. No. 11/368,979 filed Mar. 6, 2006, entitled “Power Semiconductor Switch”, by Rossano Carta et al., which claims the benefit of U.S. Provisional Patent Application No. 60/658,565 filed Mar. 4, 2005, entitled “SiC Vertical JFET Design And Method For Fabrication”, by Rossano Carta et al., which are hereby incorporated by reference.

BACKGROUND

The present invention relates to power semiconductor devices and more particularly to SiC-based JFETs.

Silicon has long been the dominant semiconductor of choice for high-voltage power electronics applications. Recently, SiC has gained the attention of designers due in large part to its wide band gap which promises much better performance than silicon particularly in high voltage applications.

SiC rectifiers, for example, high voltage Schottky diodes, are known. Diodes, however, exhibit high resistance and thus are deemed inefficient. For better efficiency, it is desirable to have a SiC based switch.

For example, for 300V-1200V applications a unipolar switch is most desirable. A MOSFET is an example of a unipolar switch. MOSFETs, however, may have reliability problems when operating at high temperatures and with high electric fields, as well as low channel mobility. These adverse characteristics may be due to the poor quality of the gate oxide.

SUMMARY

A power device according to the present invention is a SiC-based JFET. Advantageously, a SiC JFET transistor, being free of a gate oxide, is an excellent device for high temperature and high power switching applications.

A power semiconductor device according to the present invention includes a SiC body of one conductivity formed over a SiC substrate of the one conductivity, the semiconductor body including an active region having a plurality of spaced trenches each adjacent a mesa and each including a region of another conductivity formed into the sidewalls and the bottom thereof, each region of the another conductivity being deep enough so that each mesa includes a first portion of the one conductivity between two opposing regions of the another conductivity, a contact region of the one conductivity having a lower electrical resistivity than the semiconductor body formed atop each mesa, a conductive gate electrode formed adjacent and in contact with the sidewalls and the bottom of each trench, an insulation cap formed over each conductive gate electrode, a first power contact in ohmic contact with the contact regions, a second power contact in ohmic contact with the substrate.

Preferably, the conductive gate electrode comprises a polysilicon of the same conductivity as the regions of another conductivity, and each insulation cap resides within a respective trench and is substantially coplanar with the top surface of an adjacent mesa.

The present invention also relates to the design and process of fabricating a vertical JFET, which can work normally-on or normally-off, depending on the single cell geometries selected. Some of the innovative steps in the process are:

-   -   Hard mask definition (Al+SiO2 layer) to correctly etch the         trenches;     -   Multiple implant at different angles, for distributing the P         doping everywhere along the trench;     -   Doping activation using a novel Rapid Thermal Annealing step;     -   Gate and source contact planarization by filling the trench with         highly P doped polysilicon.

In a device according to the present invention, the blocking capability is created by fully depleting the channel region within the mesas with a negative bias between the gate and the source (Vgs<0). A channel region is that portion of each mesa which is of the same conductivity as the semiconductor body. The capability to block a certain voltage (600V or more) is defined by the choice of the thickness of the SiC semiconductor body and its resistivity. The device will conduct when a positive bias between the gate and the source is applied (Vgs>0) and the width of the channel region (Wch) is large enough for the vertical conduction current flow.

In a normally off device according to the present invention, blocking may be assured by pinching the channel region at a gate to source voltage equal to or lower than zero volts. The voltage necessary to cause the pinching can be determined by selecting the proper channel width, which depends on the resistivity and the depth of the regions of another conductivity, as well as the thickness of the mesas.

Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a portion of the active region of a device according to the preferred embodiment of the invention.

FIGS. 2A-2D illustrate a process for fabricating a device according to the invention.

FIG. 3 shows a cross-sectional view of a portion of the active region of a device according to another embodiment of the invention.

DETAILED DESCRIPTION

Referring to FIG. 1, a power semiconductor device according to the present invention is a JFET that includes a SiC semiconductor body 10 of one conductivity (e.g. N-type) formed over a SiC substrate 12 of the same conductivity. Note that in the preferred embodiment a SiC buffer layer 14 of one conductivity is interposed between semiconductor body 10 and substrate 12. Semiconductor body 10 includes a plurality of spaced trenches 16 each adjacent a mesa 18 and each including a region 20 of another conductivity (e.g. P-type) formed in the bottom and sidewalls thereof. Each region 20 is deep enough so that each mesa 18 includes a first portion 22 of the one conductivity and two opposing regions 24 of another conductivity. Each mesa 18 further includes a contact region 26 of the one conductivity having a lower electrical resistivity than semiconductor body 10. Formed in each trench is a conductive gate electrode 28 which is adjacent to and in direct contact with the sidewalls and bottom of the trench in which it resides. An insulation cap 30 is formed over each gate electrode 28 in order to insulate the same from first power contact 32 (referred to as source contact), which is in ohmic contact with contact region 26. A device according to the present invention further includes a second power contact 34 (referred to as drain contact) in ohmic contact with substrate 12.

Substrate 12 is preferably wafer quality SiC, buffer layer 14, semiconductor body 10 and contact region 26 are all epitaxially formed SiC. Gate electrodes 28 are preferably polysilicon bodies of the same conductivity as regions 20 (e.g. P-type), while insulation plugs 30 are preferably formed with a low density, low temperature silicon dioxide such as TEOS. In addition, first power contact 32 and second power contact 34 may be formed with aluminum or the like material, and optionally rendered solderable.

Referring next to FIGS. 2A-2D, in order to fabricate a device according to the present invention, starting with a semiconductor structure which includes substrate 12, buffer 14, semiconductor body 10 and contact region 26, first, a portion of contact region 26 is removed from termination region 36, and then SiC in termination region 36 is etched. Next, according to an aspect of the present invention a hard mask body including a silicon dioxide layer 38, and aluminum 40 is deposited. Thereafter, a photoimageable mask 42 is disposed over the hard mask body and patterned to define the areas that are to be trenched. Next, the hard mask body is patterned through the photoimageable mask 40 resulting in the structure seen in FIG. 2A. The openings in the hard mask body designate the areas for trench formation in semiconductor body 10. The broken lines define the boundaries of trenches.

After the hard mask formation, trenches 16 in active region 37, and first trench 42, and second trenches 44 are formed through dry etching. Note that first trench 42 and second trenches 44 surround active region 37. Also, note that the sidewalls of trenches 16 can be angled to be 87.degree. or higher. Next, dopants of a conductivity opposite to that of semiconductor body 10 are implanted into the sidewalls and the bottom of trenches 16 in active region 37 and first trench 42 and second trenches 44. Note that the dopants may be implanted at 90.degree. angle toward the bottom of trenches 16, 42, 44, and at other angles relative to the bottom toward the sidewalls thereof. Thereafter, the hard mask is removed, and a rapid thermal anneal step is carried out preferably at about 1650.degree. C. for about twenty minutes to form regions of another conductivity 20 in the sidewalls and the bottom of trenches 16, 42, 44 resulting in the structure shown in FIG. 2B.

Next, polysilicon is deposited in trenches 16, 42, 44. The polysilicon may be rendered conductive to have the same conductivity as regions 20 by dopant implantation (e.g. boron or aluminum) after deposition or by in situ doping while the polysilicon is being deposited. The polysilicon is then patterned by masking and etching (by plasma etching or the like) leaving gate electrodes 28 in trenches 16, and conductive electrodes 46 in trenches 42, 44 in termination region 36, as seen in FIG. 2C.

Next, a low temperature, low density field oxide such as TEOS is deposited, masked, and etched to form insulation plugs 30 over gate electrodes 28, an insulation body 48 between electrode 42 and a sidewall of trench 42, and a field insulation body 50 having a portion between electrode 46 and a sidewall of trench 42 extending over trenches 44 and the mesas therebetween. Note, also that the oxide may be patterned also for the gate and gate finger connection to gate electrodes, as well as source contact connection to contact regions 26.

Next, source metal is deposited and patterned to form first power contact 32. Note that a process according to the present invention results in a surface that is relatively flat before source metal is deposited. As a result, the source metal coverage is excellent. Optionally, the top side (also referred to as front side) of the device can be then passivated with polyimide or the like and patterned to provide access to power contact 32 and the gate contact (not shown). Thereafter, a back metal is deposited on substrate 12 to form second power contact. Preferably, a tri-metal solderable stack 52 (e.g. Ti—Ni—Ag) is formed over second power electrode 34.

Referring to FIG. 3, (in which like numerals identify like features present in the previously discussed embodiment) in an alternative embodiment, the polysilicon gate is replaced with a metal gate 54, which is electrically connected to a highly conductive contact region 56 of the same conductivity as region 20 at the bottom of each trench 16. The sidewalls of the trench are then passivated with a passivation body 58, such as silicon dioxide.

Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims. 

1. A power semiconductor device comprising: a semiconductor body of one conductivity formed over a substrate of said one conductivity, said semiconductor body including an active region having a plurality of spaced trenches each adjacent a mesa and each trench including a region of another conductivity formed into its sidewalls and bottom thereof, each region of said another conductivity being deep enough so that each mesa includes a first portion of said one conductivity between two opposing regions of said another conductivity; a contact region of said one conductivity having a lower electrical resistivity than said semiconductor body formed atop each mesa; a conductive gate electrode formed adjacent to and in contact with said sidewalls and said bottom of each trench; an insulation cap formed over each conductive gate electrode; a first power contact in ohmic contact with said contact regions; and a second power contact in ohmic contact with said substrate.
 2. The device of claim 1, wherein said conductive gate comprises polysilicon.
 3. The device of claim 1, wherein said conductive gate comprises a polycrystalline semiconductor material of said another conductivity.
 4. The device of claim 1, wherein said conductive gate comprised a polysilicon of said another conductivity.
 5. The device of claim 1, wherein said insulation cap is comprised of a low temperature depositable oxide.
 6. The device of claim 1, wherein said insulation cap is comprised of TEOS.
 7. The device of claim 1, wherein said semiconductor body includes a termination region formed around said active region, said termination region including a first trench having a region of said another conductivity formed on sidewalls and bottom thereof, a first conductive body formed only adjacent said bottom of said first trench, and an insulation body between at least said conductive body and a sidewall of said trench.
 8. The device of claim 7, wherein said termination further includes a plurality of second spaced trenches each having a region of said another conductivity formed in said sidewalls and said bottom thereof and each including a second conductive body formed therein adjacent said sidewalls and said bottom; and another insulation body disposed between first conductive body and another sidewall of said trench and extending over said second conductive bodies.
 9. The device of claim 7, wherein said first conductive body is comprised of a same material as that of said gate electrodes.
 10. The device of claim 8, wherein said first and said second conductive bodies are comprised of a same material as that of said gate electrodes.
 11. A power semiconductor device comprising: a SiC semiconductor body of one conductivity formed over a substrate of said one conductivity, said semiconductor body including an active region having a plurality of spaced trenches each adjacent a mesa and each trench including a region of another conductivity formed into its sidewalls and bottom thereof, each region of said another conductivity being deep enough so that each mesa includes a first portion of said one conductivity between two opposing regions of said another conductivity; a contact region of said one conductivity having a lower electrical resistivity than said semiconductor body formed atop each mesa; a conductive gate electrode formed adjacent to and in contact with said sidewalls and said bottom of each trench; a first power contact in ohmic contact with said contact regions; and a second power contact in ohmic contact with said substrate.
 12. The power semiconductor device of claim 11, wherein said gate electrode is comprised of a gate metal.
 13. The power semiconductor device of claim 11, wherein a passivation body is formed on sidewalls of each trench.
 14. The power semiconductor device of claim 13, wherein said passivation body is comprised of silicon dioxide.
 15. The device of claim 1, wherein said semiconductor body and said contact region are epitaxially formed SiC.
 16. The power semiconductor device of claim 11, wherein said conductive gate comprises polysilicon.
 17. The power semiconductor device of claim 11, wherein said conductive gate comprises a polycrystalline semiconductor material of said another conductivity.
 18. The power semiconductor device of claim 11, wherein said semiconductor body and said contact region are epitaxially formed SiC. 